With remarkable advances in the performance of computers and other information equipment, the volume of information that is handled by users has been constantly increasing and is now measured in gigabytes as a unit. Under these circumstances, there exists an ever-growing demand for semiconductor devices such as information storage/reproduce equipment and memories that are capable of recording at even higher densities.
To achieve higher recording densities, technologies for even finer microfabrication are required. Conventional photolithography which uses the exposure process is capable of microfabrication over a large area in one step; however, since its resolution is not finer than the wavelength of light, conventional photolithography is inevitably unsuitable for creating fine structures smaller than the wavelength of light (say, 100 nm and less). Technologies currently available for processing finer structures than the wavelength of light include exposure using electron beams, exposure using X-rays, and exposure using ion beams. However, pattern formation with an electron beam lithographic apparatus differs from patterning by one-shot exposure using such light sources as i-line and an excimer laser in that the more patterns that need be written with electron beams, the longer the time that is required for writing (exposure). Therefore, as the recording density increases, the time it takes to form a fine pattern is extended to cause a marked drop in throughput. With a view to forming patterns at a faster speed by the e-beam lithographic equipment, the development of a method for one-shot irradiation of geometric figures is underway in which combinations of variously shaped masks are subjected to one-shot exposure to electron beams; however, the e-beam lithographic apparatus that uses the method for one-shot irradiation of geometric figures is not only bulky but it also needs an additional mechanism for controlling the positions of mask to an even higher precision; this increases the cost of the lithographic apparatus, eventually leading to a higher cost for manufacturing media.
Printing-based approaches have been proposed as an alternative to the conventional exposure technologies for creating fine structures smaller than the wavelength of light. See, for example, the article titled “Imprint of sub-25 nm vias and trenches in polymers” that is carried in S. Y. Chou et al., Appl. Phys. Lett., Vol. 67, No. 21, 20 Nov. 1995, pp. 3114-3116. Nanoimprint lithography (NIL) is a technique in which a pattern of a predetermined fine structure is formed on a master by exposure to electron beams or using some other methods of creating finer structures than the wavelength of light and the master is urged under pressure against a resist-coated transfer substrate so that the fine structured pattern is transferred to the resist layer on the transfer substrate. As long as the master is available, there is no particular need to employ an expensive exposure apparatus but an apparatus in the class of ordinary printing presses will suffice to produce replicas in large quantities; hence, in comparison with the conventional methods such as exposure to electron beams, there is achieved a marked improvement in throughput whereas the manufacturing cost is significantly reduced.
When a thermoplastic resin is used as a resist material in the nanoimprint lithographic (NIL) technology, transfer is performed with the thermoplastic resin being heated under pressure to a temperature near its glass transition temperature (Tg) or higher. This approach is called a heat transfer technique and described in Yoshihiko HIRAI, Nanostructure Fabrication by Nanoimprint Technology, Journal of the Japan Society for Precision Engineering, Vol. 70, No. 10, 2004, pp. 1223-1227. The heat transfer technique has the advantage of permitting the use of general-purpose, thermoplastic resins. If a photosensitive resin is used as a resist in the NIL technology, a photocurable resin that cures upon exposure to light such as ultraviolet (UV) radiation is chosen as the resin to which the original fine pattern is transferred. This approach is called an optical transfer technique and described in Jun TANIGUCHI et al., Recent trend of nanoimprint technique, Journal of the Society for Abrasive Technology, Vol. 46, No. 6, June 2002, pp. 282-285.
In the imprint processing technology using the optical transfer technique, a special photocurable resin must be used but, on the other hand, it has the advantage of reducing the dimensional errors in finished products due to the thermal expansion of transfer printing plates or printing media. Other advantages that are related to the apparatus include elimination of the need for equipping it with a heating mechanism and providing accessories such as for performing temperature elevation, temperature control, and cooling. There is a further advantage concerning the imprint apparatus taken as a whole and that is elimination of the need for design considerations against thermal distortions, such as heat insulation.
An example of imprint apparatuses based on the optical transfer technique is described in Jun TANIGUCHI et al., Recent trend of nanoimprint technique, Journal of the Society for Abrasive Technology, Vol. 46, No. 6, June 2002, pp. 282-285. This apparatus is so designed that a quartz or sapphire mold capable of transmitting UV light is urged against a photocurable resin coated transfer substrate and irradiated with UV light from above. However, the patterned structure on the rigid quartz or sapphire mold is known to be easily damaged when the mold is pressed into contact with a rigid transfer substrate. As a further problem, in order to strip the mold fro the transfer substrate, it has been necessary to drive a wedge into the interface between the mold and the substrate. This has caused problems such as damaging of the mold or the occurrence of foreign matter in large quantities. The quartz or sapphire mold is not only very expensive but if it should be damaged, it is generally impossible to repair or reuse. In addition, if foreign matter gets seated between the mold and the transfer substrate or if any irregular protrusions from a surface of the transfer substrate create gaps between the mold and the transfer substrate, a subsequent pattern formation from the photocurable resin produces a base layer whose thickness is greater than it should be by an amount that corresponds to the created gaps. This thick base layer is impossible to remove by etching, which eventually becomes a major cause of a poorly etched final product.
With a view to solving these problems, JP 2007-55235 A proposed that a polymer stamp, or a polymeric material to which the pattern on a rigid mold has been transferred, should be substituted for the rigid mold as a secondary replica. Since the polymer stamp is flexible and elastic, it can be forcefully pressed into contact with a rigid transfer substrate, with only a small likelihood for the occurrence of unwanted accidents such as nicking of the pattern on the stamp. In addition, even if some foreign matter or protrusions occur between the stamp and the transfer substrate, the stamp itself is flexible and elastic enough to undergo a flexural deformation that allows it conform to the foreign matter or protrusions. Furthermore, the entire surface of the stamp except in the areas where the protrusions or foreign matter occurs makes intimate contact with the transfer substrate, so the thickness of the base film becomes thin enough to permit its removal by etching. Given the rigid mold, as many polymer stamps as are required can be produced, so the polymer stamps themselves can be manufactured at such a low cost that they may be discarded after being used once or several times. Notwithstanding these advantages, the polymer stamps which are at most about one millimeter thick are difficult to transport and/or align automatically and have hence involved difficulty in handling. A further problem is posed by the need to strip or separate the polymer stamp from the transfer substrate after it has been pressed into contact with the latter and subjected to photo-curing: due to its poor strippability, the polymer stamp has often remained unremoved on the transfer substrate. If this occurs, it has been necessary to strip the polymer stamp by treatment with a suitable solvent that dissolves the polymer stamp but does not dissolve the transfer substrate and the patterned resin layer on top of it.